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ProQuest Information and Learning 300 North Zeeb Road, Ann Arbor, Mi 48106-1346 USA 800-521-0600 Bean pod mottle virus ecology and management in Iowa by Rayda Kathryn Krell A dissertation submitted to the graduate faculty in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Major: Entomology Program of Study Committee: Larry P. Pedigo. Co-major professor Marlin E. Rice, Co-major professor John H. Hill Russell A. Jurenka Jon J. Tollefson Iowa State University Ames, Iowa 2002 UMI Number: 3073461 UMI UMI Microform 3073461 Copyright 2003 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 ii Graduate College Iowa State University This is to certify that the doctoral dissertation of Rayda Kathryn Krell has met the dissertation requirements of Iowa State University Signature was redacted for privacy. Signature was redacted for privacy. Signature was redacted for privacy. For the Major Program iii TABLE OF CONTENTS ABSTRACT CHAPTER 1. GENERAL INTRODUCTION 1 Dissertation Organization 1 Introduction 1 Objectives 3 Literature Review 4 Bean leaf beetle 4 Bean pod mottle virus 5 Relationship between virus, vector, and host plant 8 Current BPMV management recommendations 16 Literature Cited 19 CHAPTER 2. CHARACTERIZATION OF BEAN LEAF BEETLE (COLEOPTERA: CHRYSOMELIDAE) FLIGHT CAPACITY 33 Abstract 33 Introduction 33 Materials and Methods 35 Results 38 Discussion 39 Acknowledgements 44 Literature Cited 45 CHAPTER 3. PRIMARY INOCULUM SORUCES OF BEAN POD MOTTLE VIRUS IN IOWA 57 Abstract 57 Introduction 58 Materials and Methods 59 Results and Discussion 64 Acknowledgements 72 References Cited 73 CHAPTER 4. BEAN LEAF BEETLE (COLEOPTERA: CHRYSOMELIDAE) MANAGEMENT FOR REDUCTION OF BEAN POD MOTTLE VIRUS 87 Abstract 87 Introduction 88 Materials and Methods 91 Results and Discussion 95 Acknowledgements 103 References Cited 104 iv CHAPTER 5. SOYBEAN PLANTING DATE AS A POTENTIAL MANAGEMENT TACTIC FOR BEAN POD MOTTLE VIRUS 121 Abstract 121 Introduction 122 Materials and Methods 124 Results and Discussion 126 Acknowledgements 131 Literature Cited 132 CHAPTER 6. GENERAL CONCLUSIONS 146 Literature Cited 150 ACKNOWLEDGEMENTS 151 V ABSTRACT Bean pod mottle virus (BPMV) is a new disease problem for North Central states soybean (Glycine max (L.)) growers. The bean leaf beetle, Cerotoma trifurcata (Forster) (Coleoptera: Chrysomelidae), is the primary BPMV vector. The main objectives of this research were to understand BPMV ecology and explore possibilities for management. Bean leaf beetle, Cerotoma trifurcata (Forster) (Coleoptera: Chrysomelidae), flight capacity was measured and seventy-one percent of beetles flew <51 m and 11.76% flew >301 m. The mean of flights <51 m was 11 m. The farthest flight made by an individual beetle was 4.9 km. A survey of Iowa counties confirmed BPMV was present throughout the state. In 2002, populations of the bean leaf beetle reached the highest abundance recorded in 14 years. Three BPMV primary inoculum sources in Iowa were confirmed. Seed transmission of BPMV was 0.037%, 1.5% of overwintered bean leaf beetles transmitted BPMV, and Desmodium canadense (L.) was identified as a naturally occurring host. Application of lambda-cyhalothrin (Warrior®) (Syngenta, Wilmington, DE) after soybean emergence and again as first-generation bean leaf beetles emerged, reduced beetle densities for four and seven weeks, reduced BPMV field incidence by 26% and 27%, and reduced seed coat mottling by 10% and 19%, at locations in central and northwest Iowa respectively. Yield was 440.48 kg/ha (6.5 bu/acre) higher in the early followed by mid-season insecticide treatment at the northwest site. Four planting dates and two soybean cultivars were examined in relation to BPMV incidence. In 2000, the lowest BPMV incidence occurred in the third planting date, but the incidence was not significantly different than in the other planting dates. For all years, yield was occasionally highest (although not significantly) in the third planting date and lowest in the last planting date. In 2000, seed coat mottling was vi lowest in the third planting date. This research provides new information on BPMV ecology and experimental evidence to support management recommendations for growers. 1 CHAPTER 1. GENERAL INTRODUCTION Dissertation Organization This dissertation is organized in six chapters. The first chapter is a general introduction, which includes the objectives of this dissertation and a review of literature on the bean leaf beetle (Cerotoma trifurcata Forster) and Bean pod mottle virus (BPMV). Chapters two through five are written for submission to scientific journals. Chapter two reports the findings of a study characterizing bean leaf beetle flight capacity. In chapter three, the results of a study to elucidate the primary sources of BPMV inoculum are presented. Chapter four presents the results of an experiment to evaluate a management tactic using insecticides for bean leaf beetle and BPMV management. Chapter five reports the results of research examining the effect of soybean planting date on incidence of BPMV. Chapter six describes the general conclusions from this dissertation, followed by acknowledgements. Introduction The relationship between a virus, its host, and the environment is extremely complex. For plant viruses that are primarily vectored by insects, the relationship is even more complex because abiotic factors act on the vector in addition to the host plant. Pest management of a virus must be based on an understanding of these interactions. To dissect the relationship between virus, vector, and host plant, it is useful to begin by investigating what is known about the cycle of these interactions. If the cycle can be understood, then finding ways to break the cycle can become the focus of management. A virus that has recently become a concern in Iowa is Bean pod mottle virus (BPMV). This virus has been prevalent in the southern United States, but is becoming more 2 widespread in the North Central states (Marking 2000, Giesler et al. 2002). There are sporadic reports of the virus in the North Central states, including a report of "widespread" disease in Kansas in 1979 (Schwenk and Nickell 1980) (Table 1). Yield losses from bean pod mottle virus have been documented as high as 52% (Hopkins and Mueller 1984) and the seed coat mottling caused by the virus can reduce seed value. The bean leaf beetle, Cerotoma trifurcata (Forster) (Coleoptera: Chrysomelidae), is considered the primary vector of this virus (Newsom et al. 1980, Hopkins and Mueller 1983, Pitre 1989). Populations of the bean leaf beetle have reached the highest levels recorded in the past 14 years (Fig. 1). Large populations of the bean leaf beetle in the North Central states may be contributing to the increased incidence of BPMV. Two factors that are known to affect bean leaf beetle densities, winter temperature and planting date, have been favorable in recent years for encouraging large populations. Recently, winter temperatures in Iowa have been above normal (www.iowa- counties.com/weather/). At warmer overwintering temperatures more beetles survive for a longer period of time (Lam 1999). The other main factor thought to encourage large beetle populations is planting date. Early planting of soybeans favors early colonization of plants by beetles. Early colonization of soybean fields is beneficial for beetles because fecundity and survivorship are greater, the longer they feed on soybeans (Zeiss and Pedigo 1996). Despite this knowledge, many farmers are planting soybeans early to increase yield and avoid wet fields in April and May (Whigham 2000). The increase in early-planted soybean fields promotes large and fecund bean leaf beetle populations. High bean leaf beetle densities can cause economic damage to soybeans through feeding. Beetle feeding in combination with BPMV can produce even greater yield and seed 3 quality reductions. No study has examined the BPMV-bean leaf beetle relationship in Iowa. The following literature review describes what is known about the virus-beetle relationship and discusses areas for research into breaking the disease cycle. Objectives This research focused on accomplishing two major objectives: 1) identify sources of bean pod mottle virus inoculum in Iowa, and 2) develop management recommendations for bean pod mottle virus to reduce its impact on soybean quality and yield. 1. Source of BPMV Inoculum • Determine the extent of BPMV in Iowa. To better understand the potential impact of BPMV a map of its distribution is needed. • Determine whether BPMV is seed transmitted. Previous studies have shown that BPMV can be seed transmitted, but no study has examined seed transmission in soybean from Iowa. Seed should be evaluated to determine whether it can serve as a primary inoculum source. • Determine whether overwintered beetles can transmit virus. There is limited information on whether overwintered bean leaf beetles can transmit BPMV. This information is needed to determine how early in the season soybean plants may be exposed to BPMV by beetles in Iowa.